The primary goals of Project 3: Physics are: 1) to improve proton dose distributions through a number of studies at both the Harvard Cyclotron Laboratory (HCL) and the Northeast Proton Therapy Center (NPTC) 2) to improve the efficiency of proton therapy treatment planning, beam delivery and treatment verification; and 3) to improve the accuracy of proton beam dosimetry. with transfer of the clinical program to the NPTC we will verify the neutron shielding calculations by measurements. We will develop new methods of proton treatments using the advanced technical capabilities and control systems of the new facility. To achieve improved dose distributions at the HCL we will: continue the development of patient immobilization, stereotactic set-up techniques, and patient alignment systems.; implement double-sided range compensators; implement new beam scattering geometries to minimize beam penumbra; implement more accurate dose calculation models for treatment planning; and conduct preliminary proton radiography studies. In the NPTC we will: implement higher energy treatment beams, automatic beam energy selection, and beam energy analysis; implement isocentrically delivered treatments and multi-segment treatments; develop beam scanning to achieve large treatment fields; and develop immobilization and treatment set-up techniques for new disease sites. We will improve the efficiency of proton therapy by implementing a new 3-D treatment planning system which has faster calculation models, faster hardware, improved human interface, and beam optimization tools. Faster patient set-ups will be accomplished through the implementation of digital imaging technology and, in the NPTC, modern treatment control systems for the patient positioner and nozzle. Also in the NPTC we will implement control systems which execute and monitor complex, multi-segment treatments. We plan to conduct a number of investigations of dosimetry systems including: ionization chambers, Faraday cups, diodes, calorimetry, alanine, and radiochromic film. We will carry out Monte Carlo calculations to complement and supplement the measurements. The most important function of the Physics Project is to support patient treatments; to develop means to optimize, insofar as possible, the dose localization properties of proton beams; to make dose delivery for treatments as accurate as possible and to ensure the safety of patients and personnel.